The heart is surrounded by an autonomic nervous system (ANS) network. It is well accepted that the autonomic nervous system network creates autonomic responses in the heart. Generally speaking, a variety of nervous tissues such as nerves, ganglia, etc., are disposed on the surface of the heart, in the epicardium or myocardium of the heart, on the pericardial sac surrounding the heart sac, within, upon or beneath the pericardial sac.
This network of nervous tissue includes a variety of nerves and tissue, including the neurons, axons, dendrites, plexi, ganglia and ganglia bundles. In neurological contexts ganglia/ganglion are composed mainly of somata and dendritic structures which are bundled or connected together. Ganglia often interconnect with other ganglia to form a complex system of ganglia known as a plexus. Ganglia provide relay points and intermediary connections between different neurological structures in the body, such as the peripheral and central nervous systems.
Autonomic ganglia, which may be referred to as part of the autonomic nervous system, are those ganglia that contain the cell bodies of autonomic nerves. The autonomic nervous system (ANS or visceral nervous system) is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects, for example, heart rate, digestion, respiration rate, salivation, perspiration, the diameter of the pupils, micturition (urination), and sexual arousal. Whereas most of the actions of the ANS are involuntary, some actions, such as breathing, work in tandem with the conscious mind. The ANS is classically divided into three subsystems: the enteric nervous system, the parasympathetic nervous system and the sympathetic nervous system. Relatively recently, an important subsystem of autonomic neurones that have been named ‘non-adrenergic and non-cholinergic’ neurones (because they use nitric oxide as a neurotransmitter) have been described and found to be integral in autonomic function, particularly in the gut and the lungs. With regard to function, the ANS is usually divided into sensory (afferent) and motor (efferent) subsystems. Within these systems, however, there are inhibitory and excitatory synapses between neurones.
Other forms of ganglia include cardiac ganglia. Exemplary forms of cardiac ganglia include, for example, retro-atrial ganglion, interarterial ganglia, aortocaval ganglia, and ganglia around the Oblique Sinus of the heart. These latter ganglion include, for example, the left superior ganglia, the left inferior ganglia, the right superior ganglia, and the right inferior ganglia. There are additional ganglia around the Transverse Sinus of the heart.
Those of skill in the art will realize that still other neural and ganglia structures exist. A more complete discussion of ganglia structures and their topography can be found in Topography of Cardiac Ganglia in the adult Human Heart.